I asked Ed what he thinks of TEPCO's comment quoted in the New Scientist:

". . . for the fuel pond at reactor 4, 'the risk of recriticality is not
zero', meaning a nuclear chain reaction could restart in the rods. Quite how
this has come about is unclear."

Ed agrees this is unclear. Very unclear. Well nigh impossible. He wrote
QUOTE:


Let me explain the situation in more detail.

Two kinds of fuel are being used, uranium metal clad by zirconium (Unit,
1,2, and 4) and UO2+PuO2 clad with zirconium (Unit 3).

The spend fuel is heated by radioactive decay and must be actively cooled
using flowing water.  The new fuel is not hot.

Once power failed, the spent rods started heating the water that remained in
the ponds after being shaken by the earthquake.

Spent fuel in several of the ponds got hot enough boil water and react with
the remaining water to produce H2, which exploded and blew out the
buildings.  The shock wave from the explosion removed the rest of the
water.
Water reacts with hot Zr to produce powdered ZrO2 and with uranium to
produce powdered U3O8, which generates a lot of heat, resulting in the
observed fires.  The UO2 is only slightly reactive with water and is not the
problem.

The uranium has a melting point near 1130°C, which I doubt can be reached by
heat from radioactive decay. In any case, the rods in the ponds are in
supports that contain neutron absorbers that would prevent a critically
event. Even if the uranium melted, it would react rapidly with air and not
form a molten blob.  In other words, a blob of metal simply cannot form
outside of the isolation provided by the reactor wall.

Once the rods are hot enough to react with water, trying to cool them using
water is the worst approach because formation of U3O8 will release all the
fission products in the uranium.  The melting point of UO2+PuO2 fuel (about
2100° C) is too high to be reached under any condition outside of a reactor
and is best allowed to cool naturally.

At the present time, the best approach is to do nothing.  This will not be
done because this would admit defeat and would have to be explained to
people who simply do not understand the situation.  Consequently, the water
will eventually release most of the fission fragments and then the long
cleanup will begin.

END QUOTE

I asked what about the possibility that the fuel rod cladding has burned or
split, and the pellets are in a pile of debris at the bottom of the dry
pond. He said that is possible, but a pile of pellets still will not go
critical. The geometry will not support that. They would have to be in a
compact mound right on top of one another. Also the spent fuel "contains a
lot of poisons, which is why it was removed from the reactor." Meaning it
does not react easily. That would not apply to the fuel removed temporarily,
to be reshuffled and put back.

- Jed

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